GitLab

Support Active Directory policy in PreSigninPolicyFetcher:
Don't require a policy verification key for Active Directory.
Active Directory provided policy is not signed with any
verification key.

BUG=772372
TEST=unit_tests --gtest_filter=PreSigninPolicyFetcherTest*

Change-Id: I671713f890d1de0444f3fd462de061d8f18e373c
Reviewed-on: https://chromium-review.googlesource.com/707240
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510023}